Phased array antennas typically include an array of radiating elements backed by a ground plane, with a high dielectric medium disposed between the ground plane and the radiating element array. The backward propagating wave from the radiating element array passes through the dielectric medium, and is reflected by the ground plane back through the dielectric. The function of the dielectric is to introduce a net phase shift such that the reflected wave is coherently added to the forward propagating wave travelling away from the array. Conventional phased array antennas employing such a configuration suffer from large radiation trapping and crosstalk due to the presence of radiation emitted by the antenna to be absorbed in the high dielectric medium.
Antennas are widely utilized in microwave and millimeter-wave integrated circuits for radiating signals from an integrated chip into free space. These antennas are typically fabricated monolithically on III-V semiconductor substrate materials such as GaAs or InP.
To understand the problems associated with antennas fabricated on semiconductor substrates, one needs to look at the fundamental electromagnetic properties of a conductor on a dielectric surface. Antennas, in general, emit radiation over a well defined three-dimensional angular pattern. For an antenna fabricated on a dielectric substrate with a dielectric constant .epsilon..sub.r, the ratio of radiated power into the dielectric to radiated power into the free space is .epsilon..sub.r.sup.3/2. Thus, a planar antenna on a GaAs substrate (.epsilon..sub.r =12.8) radiates 46 times more power into the substrate than into the air.
Another problem is that the power radiated into the substrate at angles greater than EQU .THETA..sub.c =sin.sup.-1 .epsilon..sub.r.sup.-1/2
totally internally reflected at the top and bottom substrate-air interfaces. In GaAs, for instance, this occurs at an angle of 16 degrees. As a result, the vast majority of the radiated power is trapped in the substrate.
Some of this lost power can be recovered by placing a groundplane (a conducting plane beneath the dielectric) one-quarter wavelength behind the radiating surface of the antenna. This technique is acceptable provided the antenna emits monochromatic radiation. In the case of an antenna that emits a range of frequencies (a broadband antenna), the use of a groundplane will not be effective unless the dielectric constant (.epsilon..sub.r) has a 1/(frequency).sup.2 functional dependence and low loss. No material has been found that exhibits both the low loss and the required .epsilon..sub.r dependence over the large bandwidth that is desired for some antenna systems.
One way to overcome these problems is to use a three-dimensional photonic bandgap crystal as the antenna substrate. A photonic bandgap crystal is a periodic dielectric structure that exhibits a forbidden band of frequencies, or bandgap, in its electromagnetic dispersion relation. These photonic bandgap materials are well known in the art. For example, see K. M. Ho, C. T. Chan and C. M. Soukoulis, "Existence of Photonic Band Gap in Periodic Dielectric Structures," Phys, Rev. Lett. 67, 3152 (1990) and E. Yablonovitch, "Photonic Bandgap Structures," J. Opt. Soc. Am. B 10, 283 (1993).
The effect of a properly designed photonic bandgap crystal substrate on a radiating antenna is to eject all of the radiation from the substrate into free space rather than absorbing the radiation, as is the case with a normal dielectric substrate. The radiation is ejected or expelled from the crystal through Bragg scattering. This concept has been described in E. R. Brown, C. D. Parker and E. Yablonovitch, "Radiation Properties of a Planar Antenna on a Photonic-Crystal Substrate," J. Opt. Soc. Am. B 10, 404 (1993). Manufacturing methods for photonic bandgap crystals of the simple face cubic center geometry type are well known in the art. For example, see E. Yablonovitch, "Photonic Bandgap Structures," J. Opt. Soc. Am. B 10, 183 (1993).